In electrophotography processes such as xerographic processes which utilize electrostatic images in the form of surface charge patterns, an electrostatic image charge pattern is first formed on the surface of a photosensitive member which consists of a photoconductive layer overlayered on a conductive substrate. The electrostatic latent image is usually made visible by the attraction of charged particles (toner) to the image surface. The toner particles are then transferred and fixed to an image receiving sheet, e.g., paper.
The photoconductive layer of the photosensitive member (or organic photoreceptor) may contain, as active layers, a charge generation layer and a charge transport layer. Layered photosensitive members are discussed in more detail in U.S. Pat. No. 4,265,990 to Stolka et al. which is incorporated herein in its entirety by reference. Additionally, U.S. Pat. No. 4,464,450, incorporated herein in its entirety by reference, discloses a layered imaging member including a blocking layer. U.S. Pat. No. 4,666,802 to Hung et al. which is totally incorporated herein by reference discloses photoconductive elements which contain multiple active layers, e.g. a charge generation layer and a charge transport layer. The Hung et al. charge generation layer also contains a phthalocyanine pigment. Other photoconductive and photogenerating pigments are known. U.S. Pat. Nos. 4,952,471, 4,922,018 and 4,925,760, all of which are incorporated herein in their entirety by reference disclose such pigments.
Polymer binders are usually preferable components of the charge generation layer and usually essential components of the charge transport layer of organic photoreceptors. Polymer binders were usually considered to be inactive. However, it has been found that the polymer binders in the charge generation layer have significant influence on the dispersion stability and photosensitivity of the generator pigment, and the polymer binders in the charge transport layer have a significant influence on the electrical and mechanical life of the photoreceptor.
Bisphenols have been used in the production of products (e.g., polycarbonates) used as binders for charge transport molecules, and photogenerating pigments used in layered photoconductive imaging members. In U.S. Pat. No. 4,766,255 to Ong et al., which is incorporated in its entirety herein by reference, there is disclosed a process for the preparation of bisphenols which are used in the preparation of polycarbonates. The bisphenols are generally prepared by the condensation of phenols with a carbonyl compound.
In the past, polycarbonates have usually been the binders used for charge transport in organic photoreceptors. Polycarbonate binders are discussed in more detail in U.S. patent application Ser. No. 546,821 by Odell, filed Jul. 2, 1990, which is incorporated herein, in its entirety by reference. Polycarbonates have also been synthesized by polyesterification of a bisphenol and a diarylcarbonate. However, there are several conditions and potential problems which must be considered in such a process. One problem is to control the viscosity below limits tolerable by the reactor system. During the polycondensation (second) stage of the reaction, the viscosity of the polymer melt increases quite rapidly. Increases in temperature are necessary to limit the increase in viscosity as the molecular weight of the polymer increases. The problems posed by high viscosity of a polycarbonate melt are difficult to deal with and include providing adequate mixing of the molten polymer to allow for both uniform heat distribution and uniform product quality. The removal of a very viscous polymer from the reactor is more complicated than simple stirring. The application of increased temperature to obtain a lower melt viscosity has only limited success, as temperatures of 300.degree. C. and above result in a partial degradation of the polymer leading to a broader molecular weight distribution. This reduces the polymer's mechanical properties. Accordingly, there is a need for polymer resins that can be made with the advantages of melt polyesterification such as solventless polymerization and high purity product, without the disadvantages of extremely high melt viscosity and toxic byproducts.
The rate of increase in the molecular weight is controlled, at least in part, by the removal and condensation of the by-product, phenol. This control is facilitated if the phenol is kept in a liquid state, thus avoiding plugging and permitting easy measurement of the volume of removed phenol. However, under the lower pressure conditions existing in the second half of the polymerization, there is the increased possibility that the phenol will sublime and ultimately deposit in unwanted places. To avoid this, the phenol condensate must be kept cold. However, this interferes with the ability to accurately measure the volume of removed phenol, and hence makes it more difficult to control the molecular weight and to avoid plugging. In addition to these process difficulties, the phenol by-product is itself a toxin and requires special handling customary for toxic materials. The polycarbonate once produced must be purified of residual catalyst, as taught in U.S. Pat. No. 4,921,940, to produce a polymer that provides cyclic stability during xerographic processes.
The use of certain copolyesters as binder polymers in charge transport layers is also disclosed. U.S. Pat. No. 4,847,175 to Pavlisko et al. discloses the use of a norbornylidene bisphenol based polyester with crystallizable side chains. In Example 2, the norbornylidene bisphenol is copolymerized with terephthalate and azelate. The addition of a certain proportion of an essential ingredient (.alpha..omega.-hydroxyl terminated poly(ethylene 2-n-octadecylsuccinate)) is also required in the reaction. Further, the copolyester polymer binder is prepared in solution, Thus, not only is the cost of preparation increased, but also solution preparation requires polymer isolation and purification as well as disposal of the solvents.
While imaging members with various charge transporting substances, especially hole transports, including the aryl amines disclosed in the prior art, such as U.S. Pat. Nos. 4,585,884 and 4,584,253 which are incorporated in their entirety herein by reference, are suitable for their intended purposes, there continues to be a need for improved imaging members, particularly layered members, with abrasion resistant resin binders. Another need resides in the provision of layered imaging members that are compatible with liquid developer compositions. Further, there continues to be a need for layered imaging members wherein the layers are sufficiently adhered to one another to allow the continuous use of such members in repetitive imaging systems. Also, there continues to be a need for improved layered imaging members comprised of hole transport layers wherein the problems of transport molecule crystallization, e.g., bleeding and leaching, are avoided or minimized. Furthermore, there is a need for imaging members with charge transport compounds or polymers dispersed in certain polyester resin binders that are soluble in nontoxic solvents, and wherein the resulting imaging members are inert to the users thereof. A further need resides in the provision of photoconductive imaging members with desirable mechanical characteristics.
There is therefore a need to provide a bisphenol-based polyester that may be used as a polymer binder in the charge transport layer of a photoreceptor that is synthesized more efficiently, has a lower melt viscosity and a less troublesome by-product than those polymers currently in use. Compatability with a molecular dispersion of a transport molecule such as N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine does not require purification to provide a polymer with xerographic stability.